(1) Field of the Invention
The present invention relates to a material testing method for determining the activation energy for diffusion through polymers.
(2) Description of the Prior Art
Polymer coated material are frequently used in a marine environment. In these materials, a polymer such as a paint or a plastic is painted, coated or formed on a substrate. The polymer acts as a barrier preventing environmental fluids from affecting the substrate. Environmental fluid diffusing through the polymer can result in corrosion or electrical shorting. Thickness of the polymer coating is also of concern for cost, weight and geometric considerations. Thus, there is a great interest in reducing the permeability of polymer materials. This allows a thinner coating of polymer on the substrate to be protected.
Diffusion is also important for maintaining bonding of the polymer coating on the substrate. In cathodic delamination, the current model holds that water and oxygen diffuse through the polymer layer to trigger cathodic delamination. Delamination is begins by the build up of water in the space between the polymer and the substrate. This results in interfacial blisters which triggers the actual delamination or debonding.
Activation energy, Ea, is an important value for determining the rate of diffusion. Activation energy is akin to an energy barrier that must be surmounted by the reactants before a reaction can take place. The higher the activation energy, the slower the reaction proceeds at the measured temperature. Unfortunately, activation energy is often unknown for a given material or set of materials, and it requires experimental determination.
Activation energy, Ea, is typically calculated by first measuring water diffusion constants at three different temperatures for the polymer in question, and then relating them to the Arrhenius equation. This tends to be a time consuming process because the polymer samples must fully saturate with water before calculations can be made. There are also several ways to calculate diffusion coefficients, and the different methods may not produce compatible results.